The Pioneer 10 spacecraft launched #OTD in 1972.
It was a mission of firsts: passing through the asteroid belt, visiting Jupiter, crossing the orbits of Neptune and then Pluto. We detected a last, faint signal on January 23, 2003. Now it belongs to the cosmos.
Images: NASA
Pioneer 10 was also the first human-made object to achieve the escape velocity needed to carry it outside of our solar system. But that won’t happen until the late 2050s, and there won’t be a signal to notify us.
(Despite being first to achieve this escape velocity, Pioneer 10’s trajectory has it leaving the solar system well after the Voyager probes.)
The plaque is decoded via its depiction, in the upper-left corner, of the hyperfine transition in neutral hydrogen.
The hyperfine transition in Hydrogen emits a photon with a characteristic wavelength of 21cm and frequency of 1,420 MHz. Distances and frequencies shown on the plaque use these as base units.
The diagram on the left-hand side of the plaque, which also appears on the Voyager Golden Records, indicates Earth’s position relative to 14 pulsars.
The period of each pulsar, in units of inverse hyperfine frequency, is indicated in binary along its line.
The periods, which change over time in a predictable way, are given to high precision. If Pioneer 10 was ever discovered by a spacefaring species this information could be used to estimate how much time has passed since the probe’s launch.
The plaque also shows the probe’s path through our solar system, with binary depictions of each planet’s distance from the sun in multiples of 1/10 the orbital radius of Mercury.
At the time of launch the solar system comprised nine planets — poor Pluto was still in the club.
👽 1: Is this the right solar system
👽 2: Idk, their transmissions refer to only 8 planets
👽 1: [counting] But the plaque shows 9 planets
👽 2: We should visit them and ask-
👽 1: Negative, no one just changes the number of planets, let's try the next system
👽 1: We have deciphered the plaque. The location of the probe’s home planet is known to us. We should visit.
👽 2: Query. What is their technology level?
👽 1: Difficult to assess. Development of rudimentary spacecraft. But no evidence of pants.
👽 2: Okay let’s hold off on that visit.
👽 1: Jumpsuits?
👽 2: Scanning...Negative.
@mcnees fun fact: our solar system might still have 9 planets:
@Kencf618033 @mcnees The original "A Message From Earth" paper gives the list of pulsars on the plaque: https://astro.swarthmore.edu/astro61_spring2014/papers/sagan_science_1972.pdf .
The spin periods and relative directions of the pulsars are intended to specify both the Sun's position and the time the plaque was made.
However, some of the pulsar periods were not well known then and all of them have some random variation superimposed on the predictable spin-down. So eventually the map will become useless.
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